1. Field of the Invention
The present invention relates to a charge transfer apparatus for transferring charges, and an image pickup apparatus using the same.
2. Related Background Art
Conventional charge transfer elements are a CCD (Charge Coupled Device) and CSD (Charge Sweep Device; Japanese Patent Publication No. 63-38866 and Japanese Laid-Open Patent Application No. 2-63314), and are mainly applied to solid-state image pickup elements. These charge transfer elements consist of MOS diodes formed on a semiconductor, and receive signal charges in depletion layers formed near the semiconductor interface by controlling the gate electrode potentials of the MOSs. The CCD and CSD adopt different transfer methods. The CCD is constituted by forming multistage MOS diodes, a potential well is formed on each stage, and a plurality of independent signal charges are assigned to a plurality of potential wells. The gate electrode potential of MOS on the respective stage is sequentially changed to sequentially move the position of the potential well, and the signal charges are simultaneously transferred accordingly. The CSD is multistage MOS diodes or a single MOS diode. One CSD transfer path, which forms one potential well, receives only one signal charge, and thus the maximum transferable charge is large. Charges are transferred by sequentially changing the gate electrode potentials of MOSs and sequentially moving the potential barrier position in the potential well.
Noise generating during transfer in the charge transfer element is mainly a dark current generating from a semiconductor substrate or MOS interface. Both the CCD and CSD can transfer signal charges with small noise.
However, the CCD or CSD used in a solid-state image pickup element suffers the following problem.
As for an interline CCD which is most widely used among CCD solid-state image pickup elements, optical signal detection photodiodes are two-dimensionally arrayed, and signal charge transfer CCDs (vertical CCDs) are interposed between the photodiode columns. If the photodiode area is increased to increase the sensitivity in this arrangement, the CCD area must be decreased. All of the signal charges of photodiodes on one column are transferred at once to a plurality of potential wells in the vertical CCD and then transferred. The maximum charge amount per photodiode that can be transferred by the CCD is proportional to the area of one stage of MOS diodes which form one potential well. Thus, if the CCD area is decreased, the acceptable maximum charge amount is restricted. The processible maximum signal charge amount per photodiode determines the dynamic range of an image pickup apparatus. As for an interline CCD, a high-sensitivity design and a wide-dynamic-range design are contradictory to each other.
In the arrangement of a CSD solid-state image pickup element, similar to the interline CCD, CSD charge transfer paths are interposed between photodiode columns. In this case, the entire CSD region of one column receives the signal charge of one photodiode. Even if, therefore, the CSD transfer path is narrowed, the acceptable maximum charge amount is not actually restricted, and the processible maximum charge amount is determined by the maximum accumulation charge amount of one photodiode. The CSD solid-state image pickup element can simultaneously realize high sensitivity and wide dynamic range by increasing the photodiode area. However, the CSD can transfer only a set of signal charges at once, its operation is inevitably line-sequential driving, and the signal charges of photodiodes on a row selected for signal transfer must be transferred at high speed. Particularly in a movie image pickup apparatus, the signal read-out time of one row is determined by a standard, and the signal charges of a photodiode must be transferred to the output terminal of the CSD transfer path within this time.
A CSD charge transfer method for solving this problem will be explained with reference to FIGS. 1A to 1E. FIG. 1A is a schematic sectional view of a CSD, and FIGS. 1B to 1E are views of the potentials for explaining the transfer method. In FIG. 1A, a semiconductor substrate 1 is a p-type substrate. The semiconductor substrate 1, an insulating layer 2, and a gate electrode 3 made of polysilicon or the like form a MOS diode. Terminals xcfx861, xcfx862, xcfx863, xcfx864, and xcfx865 for supplying potentials to the electrode 3 supply potentials at positions apart from each other at a certain interval. A gate electrode 4 is made of polysilicon or the like. The semiconductor substrate 1, insulating layer 2, and gate electrode 4 form a MOS diode. A terminal xcfx86s supplies a potential to the electrode 4.
FIGS. 1B to 1E show the potentials of the charge transfer path, i.e., near the semiconductor interface below the MOS diode, and show transfer of a signal charge Qsig. In this case, the signal charge carriers to be transferred are electrons. In FIG. 1B, the terminals xcfx862, xcfx863, xcfx864, and xcfx865 are at potentials high enough to deplete the interface of the semiconductor substrate 1 below the electrode 3. At this time, the terminal xcfx861 is set at a lower potential than the terminal xcfx862. A potential gradient is generated between the terminals xcfx861 and xcfx862 of the electrode 3, and thus a potential gradient is generated in the transfer path below them. In this case, the electrons drift in the X direction in FIG. 1A. As the location where the electrode 3 gives a potential gradient sequentially shifts, as shown in FIGS. 1C to 1E, the signal charge drifts. The terminal xcfx86s applies a higher potential than that of the terminal xcfx865 to the electrode 4 in contact with the end of the electrode 3. A deep potential well is formed at the semiconductor interface below the electrode 4, and signal charges are finally collected to this well.
The CSD using this potential gradient can ensure a higher motion speed of signal charges than the CCD transferring charges mainly by charge diffusion. However, the CSD is difficult to realize high-speed charge transfer required for an image pickup apparatus such as a movie camera owing to the following problems.
(1) Since the capacitance between the MOS electrode and the semiconductor interface and that between the semiconductor interface and the semiconductor bulk are connected in series, only a value obtained by multiplying the potential gradient supplied to the MOS electrode by the capacitance division ratio is applied to the semiconductor interface. This capacitance division ratio is generally as low as about 0.1 to 0.3.
(2) Since the potential gradient supplied to the MOS electrode introduces an ohmic current through this portion, a large potential gradient is difficult to apply.
(3) The charge mobility at the semiconductor interface serving as a transfer path is lower than that in the semiconductor bulk.
The CSD is widely used in an infrared image pickup apparatus which must process a large amount of signal charges, but is difficult due to those problems to use in an image pickup apparatus for a visible region with a high signal read-out speed.
It is an object of the present invention to provide a charge transfer apparatus capable of high-speed signal read-out and an image pickup apparatus using the same.
It is another object of the present invention to provide an image pickup apparatus capable of high-speed signal read-out with high sensitivity and wide dynamic range.
To achieve the above object, according to an aspect of the present invention, there is provided a charge transfer apparatus comprising a semiconductor substrate of one conductivity type, a charge transfer region of a conductivity type opposite to the conductivity type of the semiconductor substrate that is formed in the semiconductor substrate and joined to the semiconductor substrate to form a diode, a signal charge input portion adapted to input a signal charge to the charge transfer region, a signal charge output portion adapted to accumulate the signal charge transferred from the charge transfer region, and a plurality of independent potential supply portions adapted to supply a potential gradient to the semiconductor substrate, wherein the signal charge in the charge transfer region is transferred by the potential gradient formed by the plurality of potential supply portions.
According to another aspect of the present invention, there is provided a charge transfer apparatus comprising a semiconductor substrate of one conductivity type, a well of a conductivity type opposite to the conductivity type of the semiconductor substrate that is formed in the semiconductor substrate, a charge transfer region of a conductivity type opposite to the conductivity type of the well that is formed in the well and joined to the well to form a diode, a signal charge input portion adapted to input a signal charge to the charge transfer region, a signal charge output portion adapted to accumulate the signal charge transferred from the charge transfer region, and a plurality of independent potential supply portions adapted to supply a potential gradient to the well, wherein the signal charge in the charge transfer region is transferred by the potential gradient formed by the plurality of potential supply portions.
According to still another aspect of the present invention, there is provided an image pickup apparatus comprising a semiconductor region of one conductivity type, photoelectric conversion portions two-dimensionally arrayed in the semiconductor region, charge transfer regions of a conductivity type opposite to the conductivity type of the semiconductor region that are interposed between respective columns of the photoelectric conversion portions and form junctions together with the semiconductor region, transfer electrodes adapted to transfer and input signal charges to the charge transfer regions, signal charge output portions adapted to accumulate the signal charges transferred from the charge transfer regions, and a plurality of independent potential supply portions adapted to supply a potential gradient to the semiconductor region, wherein the signal charges input to the charge transfer regions are transferred in a column direction by the potential gradient formed by the plurality of potential supply portions.
According to still another aspect of the present invention, there is provided an image pickup apparatus comprising a semiconductor region of one conductivity type, photoelectric conversion portions two-dimensionally arrayed in the semiconductor region, charge transfer regions of a conductivity type opposite to the conductivity type of the semiconductor region that are interposed between respective columns of the photoelectric conversion portions and form junctions together with the semiconductor region, signal amplifier portions adapted to amplify signal charges transferred from the charge transfer regions, and a plurality of independent potential supply portions adapted to supply a potential gradient to the semiconductor region, wherein the signal charges accumulated in the photoelectric conversion portions by the potential gradient formed by the plurality of potential supply portions are input to the signal amplifier portions via the transfer regions, and signals amplified by the amplifier portions are output.
The above and other objects, and features of the present invention will be apparent from the following description in conjunction with the accompanying drawings.